The objective of the proposed research is to observe the ultrastructure of active genes coding for ribosomal RNA, 5s ribosomal RNA, and specific messenger RNA's in order to obtain information regarding regulation of gene action and the processing of gene products at the level of individual genes. A variety of organisms and cell systems will be utilized, including amphibian oocytes and embryos (rRNA and 5S RNA), newborn chicks and chick embryos (rRNA and messengers for collagen, fibrous muscle proteins, hemoglobins and ovalbumin), Drosphila embryos and salivary glands (rRNA and messeangers for histones) and mammalian cell cultures (rRNA and viral genomes). Methods will include high resolution bright field and dark field electron microscopy and electron microscopic autoradiography of isolated nuclear contents and thin-sectioned nuclei and enzymatic digestion and other cytochemical probes of structural composition. It is hoped that this work will provide new information on structural aspects of gene activation and repression, the replication of transcriptionally active genes, the replication fork complexes in DNA duplication. and the interaction within RNA polymerase-nascent ribonucleoprotein-gene complexes.